1. Field of the Invention
The present invention relates to a new process for preparing halo acetals of ethylenic aldehydes of the formula (I): ##STR2## wherein: X represents a halogen atom selected from chlorine, bromine and iodine atoms;
R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 , which may be identical or different, represent a hydrogen atom or an alkyl radical containing 1 to 6 carbon atoms in a straight or branched chain, preferably a methyl or ethyl radical, or an alkenyl radical containing 3 to 6 carbon atoms in a straight or branched chain and in which the double bond is in a position other than 1-2; PA1 n is equal to 0, 1, 2, 3 or 4, with the understanding that, when n is greater than 1, each R.sub.4 and each R.sub.5 may be identical or different; and R.sub.7 each represent an alkyl radical containing 1 to 6 carbon atoms in a straight or branched chain, preferably a methyl or ethyl radical, or, together, form an alkylene radical R'.sub.7 containing 2 to 6 carbon atoms in a straight or branched chain, preferably a --CH.sub.2 --CH.sub.2 -- radical, and optionally substituted with a hydroxyl radical or an alkyloxy radical containing 1 to 4 carbon atoms.
It is specified that the term halogen as used hereinafter encompasses only chlorine, bromine and iodine.
2. Description of the Related Art
Halo acetals of ethylenic aldehydes are organic compounds which are especially useful as organic synthesis intermediates. Thus, according to the general process described in British Patent 1,396,622, they may be used for introducing an .alpha.,.beta.-ethylenic aldehyde unit into a mono or polyene residue by reaction with a polyene sulphone in the presence of an alkaline agent, the sulphone resulting from this condensation then being desulphonated with the formation of a further double bond.
It is known that 7-halo acetals of .alpha.,.beta.-ethylenic aldehydes may be prepared by the haloalkylation of a 1-alkyloxy-1,3-diene, by the action of an N-halosuccinimide in the presence of an alcohol according to the process described by S. M. Makin et al., J. Gen. Chem. URSS, 32:1088 (1962). The drawback of this process, however, is that the starting diethylenic ethers are difficult to prepare. The starting ethers are generally prepared by the treatment of acetals of .alpha.,.beta.- or .beta.,.gamma.-ethylenic aldehydes at high temperature in the presence of catalysts, these initial starting materials being in and of themselves difficult to synthesize. Although the method of Makin is also applicable to the synthesis of .omega.-halo acetals of aldehydes containing a system of conjugated double bonds, the preparation of such compounds by this method presents very great problems because of the difficulty of synthesizing the necessary starting materials.
Another process is disclosed in U.S. Pat. No. 4,100,201, wherein .omega.-halo acetals of ethylenic aldehydes are prepared by the halo-genation of an ester, preferably an acetic ester, corresponding to the ethylenic aldehyde. According to the paper by H. J. Hagemeyer published in Ind. Eng. Chem. 41: 2920 (1949), this ester is prepared by exchange between the aldehyde and isopropenyl acetate. During the regeneration of the aldehyde after hydrolysis of the ester, acetic acid is liberated, thereby making it impossible to regenerate the expensive starting material, i.e., isopropenyl acetate. Thus, this process, like the Makin process above, is uneconomical.
A further process is disclosed in U.S. Pat. No. 4,087,405, wherein .omega.-halo acetals of polyene aldehydes are prepared by the reaction of a halogen cation with an enoxysilane. Enoxysilanes are compounds which are difficult to obtain from an industrial standpoint and, as a result, are expensive starting materials.